In 3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP LTE)-Advanced, a technology called Carrier Aggregation (CA) has been studied in order to increase bandwidth. CA is a communication technology that enables a plurality of different frequency bands to be used. A communication area where communication is performed by using a single frequency band is referred to as a cell. Therefore, CA may be described as a communication technology for simultaneously using two or more of all the cells of a base station.
When CA is performed, a single terminal belongs to two or more of all the cells of a base station. In this case, the base station assigns, to the terminal, an identification number common to the two or more cells from a predetermined number of identification numbers for identifying terminals. Then, the terminal performs communication using the two or more cells by using the identification number assigned by the base station. In 3GPP, the identification number for identifying a terminal is referred to as a Cell-Radio Network Temporary Identifier (C-RNTI) number. A range of the C-RNTI numbers defined by 3GPP is from 1 to 65535. Patent Literature 1 is an example of a document that proposes a technology for assigning a C-RNTI at the time of CA.    Patent Literature 1: Japanese National Publication of International Patent Application No. 2013-509055
When communication other than CA is performed, a single terminal belongs to any of all the cells of a base station. The base station assigns an arbitrary C-RNTI number to the terminal from the predetermined number of C-RNTI numbers. The arbitrary C-RNTI number is assigned independently for each cell. Therefore, when CA is performed after communication other than CA, C-RNTI numbers assigned to the terminal may conflict with each other.
In this regard, to prevent the C-RNTI numbers from overlapping between all the cells of a base station, it may be possible to divide the predetermined number of C-RNTIs for each of the cells, shift the ranges of the divided C-RNTIs among the cells, and fix the ranges of the C-RNTIs corresponding to the respective cells. However, in this method, the range of the C-RNTI numbers for each of the cells is fixed, so that the C-RNTI numbers for each of the cells may be insufficient depending on the total number of the cells of the base station. As a result, the C-RNTI numbers to be assigned to terminals may be depleted. The depletion of the C-RNTI numbers in the method of fixing the range of the C-RNTIs corresponding to each of the cells may occur based on underlying assumptions as follows:                a functional unit that assigns a C-RNTI number is provided in each cell;        an assigning unit in each cell assigns a C-RNTI number within the range of divided C-RNTI numbers; and        a C-RNTI number assigned in each cell is also used in the same manner when communication using other cells is performed with implementation of CA.        